Reconstruction of the water table from self-potential data: a bayesian approach

Integration of ERT, IP and SP Methods in Hard Rock Engineering

Investigation of a hard rock site for the development of engineered structures mainly depends on the delineation of weathered and unweathered rock, and the fractures/faults. Traditionally, borehole tests are used in such investigations. However, such approaches are expensive and time-consuming, require more equipment, cannot be conducted in steep topographic areas, and provide low coverage of the area with point measurements only. Conversely, geophysical methods are non-invasive, economical, and provide large coverage of an area through both vertical and lateral imaging of the subsurface. The geophysical method, electrical resistivity tomography (ERT), can reduce a significant number of expensive drilling tests in geotechnical investigations. However, a geophysical method alone may provide ambiguity in the interpretation of the subsurface, such as electrical resistivity cannot differentiate between water and clay content. Such uncertainty can be improved by the integration of ERT with induced polarization (IP). Similarly, self-potential (SP) can be integrated with other geophysical methods to delineate the groundwater flow. In this contribution, we integrated three geophysical methods (ERT, IP and SP) to delineate the weathered and unweathered rock including the weathered/unweathered transition zone, to detect the fractures/faults, and to map the groundwater flow. Based on ERT, IP and SP results, we develop a geophysical conceptual site model which can be used by site engineers to interpret/implement the findings for build-out. Our approach fills the gaps between the well data and geological model and suggests the most suitable places for the development of engineered structures in the hard rock terrains.

Joint geophysical prospecting for groundwater exploration in weathered terrains of South Guangdong, China

Groundwater occurrence in a hard rock terrain is strongly controlled by the weathered/fractured zones. However, delineation of such zones is a challenging task given their structural heterogeneity. Traditionally, large numbers of well tests are conducted to assess the subsurface formation. But, such tests suffer from efficiency in terms of cost, time, and data coverage. Non-invasive geophysical methods can be the best alternative of expensive drilling methods. However, a geophysical method alone is ambiguous to interpret the highly heterogeneous subsurface formation. In this study, joint application of electrical resistivity tomography (ERT), magnetic method, and joint profile method (JPM) was conducted for groundwater exploitation in a weathered terrain of South Guangdong, China. ERT, magnetic, and JPM data were acquired along different geophysical profiles via a variety of survey parameters. The interpreted 2D models of electrical resistivity and magnetic data coupled with the local accessible boreholes and hydrogeological information constrain the subsurface geologic formation into four discrete layers with specific electrical resistivity range, i.e., topsoil cover, highly weathered saturated layer, semi-weathered saturated layer, and un-weathered substratum. Incorporation of JPM (ER, SP, and IP methods) with ERT and magnetic models reveal three faults (F1, F2, and F3) and several saturated intense fractures/discontinuities. The groundwater reserves associated with the weathered/fractured rock were estimated via hydraulic parameters, namely hydraulic conductivity and transmissivity. The results suggest that high-yield groundwater resources are found within the weathered/fractured zones. Geophysical results of this joint application fit pretty well to the local hydrogeological data of the study area. Our novel approach reduces any ambiguity caused in the geophysical interpretation and provides clearer insight of the subsurface formation with more confident solutions to the most challenging problems of the hard rock sites. This hydrogeophysical study provides important contributions to groundwater exploration in areas where weathering has significant effects on the hard rock aquifer system. Compared with traditional methods, this approach is more advantageous for assessment of groundwater resources in hard rock terrains.

Modeling transient streaming potentials in falling-head permeameter tests

We present transient streaming potential data collected during falling-head permeameter tests performed on samples of two sands with different physical and chemical properties. The objective of the work is to estimate hydraulic conductivity (K) and the electrokinetic coupling coefficient (Cl ) of the sand samples. A semi-empirical model based on the falling-head permeameter flow model and electrokinetic coupling is used to analyze the streaming potential data and to estimate K and Cl . The values of K estimated from head data are used to validate the streaming potential method. Estimates of K from streaming potential data closely match those obtained from the associated head data, with less than 10% deviation. The electrokinetic coupling coefficient was estimated from streaming potential vs. (1) time and (2) head data for both sands. The results indicate that, within limits of experimental error, the values of Cl estimated by the two methods are essentially the same. The results of this work demonstrate that a temporal record of the streaming potential response in falling-head permeameter tests can be used to estimate both K and Cl . They further indicate the potential for using transient streaming potential data as a proxy for hydraulic head in hydrogeology applications.

Self-potential monitoring of a crude oil-contaminated site (Trecate, Italy)

We present a multidisciplinary approach for characterization of a crude oil-contaminated site (Trecate, Italy), integrating geophysical data, such as subsoil electrical potential (in millivolts) and electrical resistivity (in ohm meters) distribution, with hydrogeological and bio-chemical data. Self-potential measurements have been evaluated together with active geoelectrical measurements and hydrological information, to provide spatial and temporal information about the self-potential sources and their possible correlations with the contamination state of the subsoil. Three self-potential surveys (March 2010, October 2010, and March 2011) were conducted at the site, both in the contaminated and uncontaminated regions. The obtained self-potential maps show large time-lapse differences in correspondence of the contaminated area, with positive electrical potential values (up to 50 mV) in spring surveys and an electrical potential dipolar distribution in October (2010) survey (amplitude from -15 to 25 mV). To understand the origin of the measured self-potential signals, a model using vertical dipolar electrical sources was built, taking into account the electrical resistivity distribution deduced from electrical resistivity tomography. The self-potential source identification allows the Trecate contamination state to be better delineated. In particular, two self-potential contributions are superimposed: the electrokinetic mechanism is predominant in spring, while the redox mechanism represents the most important contribution in autumn.